In many tests in which pigments are examined, the pigments must be made into pastes. Such tests are described in various standard specifications of the German Industrial Standards Committee, e.g. DIN 53165 (Determination of the Relative Scattering Power of White Pigments), DIN 53191 and 53192 (Determination of Lightening Power of White Pigments), DIN 53204 (Determination of the Relative Tinting Strength of Colored Pigments), DIN 53238 (Examination of Dispersibility), parts 11 and 20, and DIN 53239 (Determination of Fineness-of-Grind during Dispersion in an Automatic Muller).
The pastes are ground in a plate-type paint grinding apparatus. The main features of the apparatus include two circular mulling plates, preferably made of glass or covered with glass plates, which are positioned coaxially one above the other in a horizontal attitude. A drive gear positioned below the lower plate cooperatively engages the lower plate to revolve that plate in one direction about its central axis. The upper plate does not revolve about its central axis; it can, however, be folded or raised upwards to feed the paint grinding apparatus. The paint grinding apparatus is further equipped with a mechanism to apply a specified load which can, if required, be exerted by the upper plate on the lower plate. Preferably, one of the two plates, usually the upper one, can be cooled or heated. Such a plate-type paint grinding machine is, for instance, known under the name of "Automatic Muller," manufactured by J. Engelsmann AG, D-6700 Ludwigshafen/West Germany, and sold, for instance, by A. M. Erichsen, G.m.b.H., D-5870 Hemer-Sundwig/West Germany; cf. their brochure "Prufgerate fur die Lack+Farben herstellende und verarbeitende Industrie Lieferprogramm 1964" (Testing instruments for the paint making and processing industry, 1964 production program), page 6, type No. 354.
The above apparatus employed heretofore requires the following sequence of operation. The paste is made in such a manner that first, with the upper plate folded upwards, the exactly defined quantities of a paste and the pigment to be examined are placed on the lower plate and mixed by hand with a spatula. Then the upper plate is put down, with the prescribed load applied if required, and the lower plate is set in motion. After a specified number of revolutions of the lower plate operation has to be interrupted and the paste homogenized by hand with the aid of a spatula. After the same procedure has been repeated, if needed several times, the mixture is collected from the paint grinding machine and processed further for testing.
Interruption of the machine during grinding of the paste is required because the mixture tends to separate while the lower plate is in motion so that, in places, higher pigment concentrations will occur. These inhomogeneities must be removed by hand. This procedure is time consuming and, because of the manual work involved, expensive. These disadvantages provide serious drawbacks when testing a large number of pigment or paint samples.
The present invention avoids the foregoing disadvantages, particularly the interruption of the machine operation and eliminates the need to homogenize the paste by hand during the periods of interrupted machine operation.